Auschwitz-Birkenau - a pair of the most terrible Nazi concentration camps in all of Europe. Poland had a long history of peace between Catholic and Jews and for that reason had one of the largest Jewish populations in all of Europe. I heard the figure (from the Rick Steves' video) that only 10 percent of Poland's Jewish population survived the war. St. Maximillian Kolbe and St. Edith Stein (St. Teresa Benedicta) were killed here. Elie Wiesel, the 1986 Nobel Peace Prize recipient, wrote of his life at Auschwitz as a teenager in his bestseller Night (the book is suitable for older teens and adults). It was also from Auschwitz that Oscar Schindler saved more than 1200 Jews on his famous "list". (The movie is amazing, but very difficult to watch and best suited for older teens or adults - younger children could read A Place to Hide: True Stories of Holocaust Rescues by Jayne Pettit).

UPDATE: There were actually three parts to Auschwitz - Auschwitz I, Auschwitz II/Birkenau and Auschwitz III.